The equilibrium molecular structures of cross-conjugated alkenes with particular reference to [3]- and [4]-dendralene from ab initio studies of various conformers

Abstract

A study of the equilibrium structures and relative energies for the conformers of 3-methylenepenta-1,4-diene (MPD) and bis(3,4-methylene)-1,5-hexadiene (MHD) have been obtained by SCF and MP2 correlated methods; a double zeta+polarisation basis set (DZP) was used. Both planar and non-planar forms were evaluated for a variety of point groups. The force constants for each conformer were obtained, in order to determine whether the minima were saddle points rather than genuine minima owing to the constraints of molecular symmetry. For both molecules, all the non-planar forms studied were of lower energy than the best planar-forms. Indeed the planar structures are all saddle-points between the interconverting pairs of non-planar forms, as shown by one or more negative vibration frequencies for the planar molecules. The relative energies and structures of the different conformers of MPD and MHD are almost identical between the SCF and MP2 series, the MP2 calculations merely leading to slight extensions of the C=C bonds, minor changes in dihedral angles, and small changes otherwise. Because of the H⋯H interactions, all the planar forms showed considerable in-plane angle bending, and small but consistent H–C bond compressions were also obtained in the most hindered cases. The lowest energy conformer of MPD is a gauche-trans conformer, with symmetry C 1, but a second conformer of trans-trans type and C 2 symmetry lies only 6 kJ mol −1 above. The C 2 non-planar conformer of MHD containing (basically) trans-butadiene units is the lowest energy by a significant amount, but several trans–cis and cis–cis conformers were all identified as potential species for gas phase mixtures.